Hull

The term hull refers to the body or frame of a ship or starship. Warp-worthy vessels must have a hull able to withstand extreme accelerations. The hull of a Starfleet vessel was usually separated into two parts: The primary and secondary hulls.

Hull stress was a condition where, on a starship, space station or other installation, the hull encounters a form of stress. This could be caused from a form of pressure pulling against the ship, such as an object pulling against another. Hull stress, if critical enough, can lead to a hull breach.

When the USS Voyager was caught in a quantum singularity in 2371, they found what they thought to be another ship caught further in the singularity. When Voyager attempted to lock on the ship with a tractor beam, the singularity started to pull the ship in. The ship was put into full reverse impulse, but with the tractor beam still on, this caused hull stress. (VOY: "Parallax")

Microfractures were microscopic cracks in the hull of a starship that, while invisible in the naked eye, could widen over time if something aggravated it and cause more drastic problems. The term also has a medical context.

The structural integrity of a material was diminished when microfractures were present. This could have dangerous, even deadly, consequences when the deterioration occurred in a ship's hull or other structure which provided containment, shelter, or life support, or in a living tissue. However, under certain conditions, a limited amount of microfracturing could be tolerable. (VOY: "Extreme Risk")

Some microfractures could be sealed by an engineering team, yielding a temporary or permanent repair. Other microfractures required the rebuilding or replacement of the structure. (ENT: "Desert Crossing"; VOY: "Caretaker")

The term hull breach refers to the failure of structural integrity where the shell surrounding said pressurized environment breaks, stresses or develops a gap for any reason. In space-faring vessels, this is usually catastrophic. The pressure of a contained atmosphere forcing its way outward in an explosive decompression can cause further damage to the area, and also have the added effect of endangering or killing any lifeform present. Since the pressure is internally generated, Lieutenant CommanderData stated to CommanderWilliam T. Riker in 2364 that being "blown out" was probably a more accurate term than being "sucked out." (TNG: "The Naked Now"; VOY: "Deadlock")

The decompression Data described was caused by an opened hatch, not an actual hull breach, but the effect is essentially the same.

Most hull breach dangers are from pressurized material contained within a vacuum, or lower pressure, environment. However, it is just as possible that the pressurized vessel could be surrounded by high pressure gases or even liquid, causing an implosion, forcing the surrounding environment into a vessel. The USS Defiant experienced such breaches in 2372 when trapped in the atmosphere of a gas giant. (DS9: "Starship Down")